ash: remove arithmetic expansion collapsing at parse time

Collapsing arithmetic expansion is incorrect when the inner arithmetic
expansion is a part of a parameter expansion.

Test case:
   unset a
   echo $((3 + ${a:=$((4 + 5))}))
   echo $a
Old result:
   12
   (4 + 5)
New result:
   12
   9

Based on commit bb777a6 from git://git.kernel.org/pub/scm/utils/dash/dash.git
by Herbert Xu

function                                             old     new   delta
readtoken1                                          3180    3163     -17

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash.c

@@ -11316,9 +11316,9 @@ readtoken1(int c, int syntax, char *eofmark, int striptabs)
 				parenlevel--;
 			} else {
 				if (pgetc() == ')') {
+					c = CTLENDARI;
 					if (--arinest == 0) {
 						syntax = prevsyntax;
-						c = CTLENDARI;
 					}
 				} else {
 					/*
@@ -11809,18 +11809,12 @@ parsearith: {
 	if (++arinest == 1) {
 		prevsyntax = syntax;
 		syntax = ARISYNTAX;
-		USTPUTC(CTLARI, out);
-		if (dblquote)
-			USTPUTC('"', out);
-		else
-			USTPUTC(' ', out);
-	} else {
-		/*
-		 * we collapse embedded arithmetic expansion to
-		 * parenthesis, which should be equivalent
-		 */
-		USTPUTC('(', out);
 	}
+	USTPUTC(CTLARI, out);
+	if (dblquote)
+		USTPUTC('"', out);
+	else
+		USTPUTC(' ', out);
 	goto parsearith_return;
 }
 #endif

shell/ash_test/ash-vars/var-do-not-collapse-arithmetic-expansion-at-parse-time.right

@@ -0,0 +1,2 @@
+12
+9

shell/ash_test/ash-vars/var-do-not-collapse-arithmetic-expansion-at-parse-time.tests

@@ -0,0 +1,3 @@
+unset a
+echo $((3 + ${a:=$((4 + 5))}))
+echo $a